This invention relates to a radio telephone system and method for use in a vehicle, for example a motor car, more particularly a radio telephone system capable continuing communication by changing channels when a vehicle moves from a radio zone to another radio zone.
In a prior art moving radio telephone system, for example, in a system disclosed in U.S. Pat. No. 4,144,409 a service area is divided into a plurality of many small radio zones each installed with a base station (BS). Several radio zones are grouped into a control zone in which a radio control center cc is installed. The radio control center is connected to a conventional telephone exchange office (JEO).
In the vehicle radio system of the type described above, since the service area is divided into a number of small radio, zones, there are many chances in which a vehicle moves into another radio zone from one radio zone during speech. At time of passing through the boundary the speech condition becomes poor. For this reason, it is necessary to change the speech channel to the speech channel of the new radio zone in order to maintain the talking. According to a prior art system, due to the channel switching, the talking is temporarily interrupted whereby the vehicle and a stationary telephone set would be brought a soundless or talking impossible state. As a consequence, the talkers can not judge whether such state is caused by the channel switching or termination of the talking.
Regarding such vehicle radio telephone system there are the following publications:
(1) Sakamoto et al "Mobile Unit for 800 MHz Band Land Mobile Telephone System", Review of the Electrical Communication Laboratories, Vol. 25, Nos. 11-12. pp 1231-1244, November-December, 1977,
(2) Nagatsu et al "Base Station Radio Equipment for 800 MHz Band Land Mobile Telephone System", Review of the Electrical Communication Laboratories, Vol. 25, Nos. 11-12, pp 1245-1254, November-December, 1977,
(3) Kamata et al "800 MHz Band Land Mobile Telephone Radio System" Review of the Electrical Communication Laboratories, Vol 25, Nos. 11-12, pp 1157-1171, November-December, and
(4) Yoshikawa et al "800 MHz Band Land Mobile Telephone Control System" Review of the Electrical Communication Laboratories, Vol. 25, Nos. 11-12, pp 1172-1190, November-December, 1977.